42 



TROWBRIDGE. 



source of Rontgen rays, and its shadow appears on the screen, 

 but as the bones absorb the rays to a much greater extent than 

 the flesh, they are projected as dark shadows, while the shadow 

 of the flesh is so faint that it can hardly be seen. Thus, the 

 bones of the hand appear distinctly in outline ; the effect being' 

 somewhat similar to the silhouettes of ordinary light 



Fig. 14. Edison Fluoroscope. 



Most of the appliances for obtaining visual effect of Rontgen 

 rays by the use of the fluorescent screen are based on the dis- 

 coveries of Professor Rontgen, for it was he who first discovered 

 that fluorescent substances became luminous under the effects 

 of the X-rays. He studied both this phenomenon and the 

 effect of the rays upon the photographic negative in his famous 

 research, and obtained shadows of the bones of the hand by 

 the use of the fluorescent screen, as well as the more perma- 

 nent shadow-pictures by the photographic process. 



The fluorescent effect of certain materials when subjected to 

 the influence of Rontgen rays was, however, partly anticipated 

 by certain observations of Dr. Lenard, of Bonn. This investi- 



